The inner ear vestibular system is composed of two laterally symmetric sets of end organs (see for example Chapters 3 and 4 in Wilson, V. J., Melvill Jones, G., Mammalian Vestibular Physiology, Plenum Press, New York, 365 pp (1979). Each ear contains fine spatially specific end organs for sensing head accelerations. In each ear three semicircular canals sense angular, accelerations in three approximately orthogonal axes. The utricular otoliths sense the sum of gravity and linear head accelerations in a plane inclined approximately 30 degrees from horizontal. Function of the saccule is less understood but is believed to include gravity and linear acceleration along an approximately vertical axis. Thus, individual vestibular end organs are involved in maintaining different components of posture and equilibrium. The horizontal canals are used primarily to control horizontal plane eye and head movements, while the vertical canals and otoliths help maintain front-to-back and side-to-side balance of the head and trunk. The spatial and functional specificity within the vestibular system provides an opportunity for selectively determining the extent of pathology of individual end organs by observing both head, eye, and body responses to vestibular stimulation.
It is also known that the canal and otolith end organs sense different frequency components of linear and angular motion. Canals sense angular acceleration frequencies over the range of 0.1 to approximately 5 Hz, while the otoliths sense lower frequency linear accelerations in the range of 0 to 0.1 Hz (Meary, J. L., the vestibular system and human dynamic space orientation, NASA CR-628 (1966)). Thus, the use of frequency selective signals is another possible means for isolating the function, of individual vestibular end organs.
Vestibular pathology frequently affects only a portion of the vestibular end organs, sometimes in one ear and other times distributed either equally or unequally among the organs of the two ears (see, for example, Schuknecht, H. F., Pathology of the ear, Harvard University Press, Cambridge, Mass. (1974)). While, the treatment of choice for the patient with vestibular pathology depends on the distribution and extent of involvement among the ten end organs, the symptoms of the individual patient frequently do not reveal which organs are affected. Hence, objective methods for assessing the function of individual vestibular end organs are essential to the comprehensive vestibular examination.
To test vestibular functions head acceleration, stimuli can be imposed with precise time course, amplitude, and spatial specificity. However, precise acceleration stimuli cannot be effectively used to test vestibular end organs individually, because the lateral symmetry of the two inner ears means that acceleration in any one axis will always excite end organs in both ears. Hence, several alternative means for selective stimulation of end organs in a single ear have been developed using non-physiologic inputs: (1) Using so called "galvanic" vestibular stimulation, end organs of one ear can be electrically excited by passing small currents between two or more surface electrodes affixed to the mastoid bone of the ear or other locations on the head (see, for example, Nashner, L. M., Wolfson, P., Influence of head position and proprioceptive cues on short latency postural reflexes evoked by galvanic stimulation of the human labyrinth, Brain Research 67: 255-268 (1974)). (2) The so called "caloric" stimulus excites the horizontal semicircular canal end organ of one ear by creating a thermally induced pressure gradient within the horizontal canal (see, for example, Dayal, V. S., Farkashidy, J., Kuzin, B., Clinical evaluation of the hot caloric test as a screening procedure, Laryngoscope 83: 1433 (1973)). (3) In some instances changes in air pressure between the external canal and middle ear spaces of one ear can excite one or more end organs in that ear (see for example Daspit, C. P. Churchill, D., Linthicum, F. H., Diagnosis of perilymph fistula using ENG and impedance, Laryngoscope 90: 217-223 (1980).
Various attempts have been made to use the "galvanic" vestibular stimulus as a clinical diagnostic tool (for examples, Ishihara, A., Galvanic stimulation of the labyrinth, Jap. J. Otol. Tokyo 24: 482 (1918); Fischer, J. J., Galvanic reaction, The labyrinth, Grune and Stratton Inc, New York (1956); Pfaltz, C. R., Koike, Y., Galvanic test in central vestibular lesion, Acta, Otolarying. (Stockh) 65: 161 (1968)). In this test the vestibular end organs are selectively stimulated by passing small electrical currents between electrodes placed in different configurations on the mastoid bones. Placing one electrode on each mastoid bone stimulates receptors in both inner ears in opposite directions, while two electrodes placed on a single mastoid bone stimulate receptors of one ear selectively. While the time course, amplitude, and frequency of electrical current stimuli can be precisely controlled, the distribution of stimulation among the 5 end organs of the stimulated ear can be accomplished only to a limited degree by altering the placement of the electrodes. Responses to electrical vestibular stimulation can be monitored as movements of the eyes (for example, Hozawa, J., A clinical consideration on the nature of electrically stimulated nystagmus, Otologica, Tokyo 33: 939 (1961)) or in the standing subject as body swaying (for example, Coats, A. C., Stolz, M. S., the recorded body sway response to galvanic stimulation of the labyrinth, Laryngoscope 79: 85 (1969); Coats, A. C., Effects of varying stimulus parameters on the galvanic body sway response, Ann. Otol. 82: 96 (1973)).
Electrical stimulation of the vestibular receptor organs is a potentially useful clinical diagnostic method, because it can be used to quantify receptor function of one ear at a time and because the time course and frequency of stimulation can be precisely controlled. Electrical vestibular testing for this or any other purpose, however, is not currently a standard of practice in the clinic. This is because relatively large currents are required to produce postural or eye movement responses when the subject is tested under passive seated or reclining positions. Large stimulus levels can cause the patient significant discomfort. Furthermore, the resulting eye movement and sway responses are small, making the repeatability and reliability of the resulting measurements poor.
The use of "caloric" stimulation is already a standard practice in the currently used clinical vestibular examination, and several manufacturers produce caloric stimulation devices for this purpose. With the caloric test, the patient assumes a passive reclining position on a chair or bed. The head is positioned tilted 30 degrees back so that the plane of the horizontal canals is oriented roughly vertical. Then, hot or cold water is introduced to one ear and the amplitude and duration of nystagmoid eye movement responses are observed subjectively or measured using electronystagmography (ENG's). Alternatively, bilateral stimuli can be imposed by introducing thermal stimuli of either the same or opposite temperatures to the two ears simultaneously.
The caloric test is currently used by clinicians to identify asymmetries in function between the two ears. The sensitivity and specificity of this method is limited, however, for several reasons. First, with currently available methods, the thermal input stimulates only the horizontal canals and therefore does not detect asymmetries involving the vertical canals or otoliths. Second, the amplitude and frequency of the thermal stimulus cannot be controlled precisely, because heat conduction through the temporal bone is slow and varies among patients. Thus, the time course of the thermal vestibular stimulus is also slow and tests only the lowest frequency component of the horizontal canal response. Third, patients frequently become dizzy, motion sick, or nauseous with the caloric test.
The third method for selectively stimulating one or more vestibular end organs is to alter the pressure between the external and middle ear spaces. As with the caloric vestibular test, pressure stimuli are introduced with the patient in a passive seated or standing position.